(1) Field of the Invention
The present invention relates to a magnetic core for electronic devices which is composed of a Co-base amorphous alloy ribbon wound into a toroidal shape. More particularly, it relates to a magnetic core which suffers only low core loss during its operation. (2) Description of the Prior Art
Because of recent development of electronic devices, switching power sources carrying a magnetic amplifier have been being used more and more widely used. A main portion constituting a magnetic amplifier is a saturable reactor comprising a magnetic core including a metal ribbon wound into a toroidal shape. Magnetic core materials provided with a high saturation flux density Bs and a high rectangular ratio Br/Bs where Br represents a residual magnetic flux density are now desired to use for cores of the saturable reactors in switching power sources. Heretofore, there have been used 50% Ni-Permalloys, 80% Ni-Permalloys, etc. But these alloys fail to meet the recent high frequency requirements of switching power sources which have increasingly high performance with reduced size and weight. Specifically, 50% Ni-Permalloys and 80% Ni-Permalloys suffer enormous core loss when these alloys are used in a high frequency magnetic field. Consequently, magnetic core materials excellent in high frequency characteristics have been required.
A proposal has been made to provide a magnetic core comprising a heat-treated Co-base amorphous alloy ribbon wound into a toroidal shape to overcome the above-stated disadvantages of 50% Ni-Permalloys and 80% Ni-Permalloys. One may refer to U.S. Pat. No. 4,473,417. One method of heat treatments applied to Co-base amorphous alloy is to quench it after keeping it at a temperature higher than its Curie temperature (Tc).
Although amorphous alloys processed by the above heat treatment may have a low initial core loss, such alloys suffer a low rectangular ratio (Br/Bs), and a big disaccommodation of core loss. As a result, saturable reactors made of such materials have a tendency to become uncontrollable because of excessive heat generation which also cause undesirable effects on the neighboring elements in a switching power source.
It was also proposed to treat a Co-base amorphous alloy ribbon at a temperature below its Curie temperature where the alloy remains magnetic, from the aspect that a Co-base amorphous alloy is easily provided with an inductive magnetic anisotropy, because an amorphous state is a metastable state metallurgically. In the above-mentioned case, an amorphous metal alloy ribbon having a low core loss is produced by a rapid quench of a melt, then heat-treated at a temperature below its Curie temperature in a magnetic field in order to enhance its rectangular ratio Br/Bs.
However, it was recently recognized that a toroidal core comprised of an amorphous alloy which was heat-treated by the above-mentioned method has a big rectangular ratio and a small disaccommodation of core loss, but the core loss thereof is bigger than those of amorphous alloy heat-treated at a temperature above its Curie temperature.
Now, a toroidal core comprised of an amorphous metal ribbon provided with better characteristics than those of the previous cores is desired. Before this invention, it of the generally believed core is desired. Before this invention, it was the general idea that a Co-base amorphous metal provided with a high rectangular ratio (Br/Bs) has a big core loss, although it has a small disaccommodation of core loss, and on the contrary one provided with a low rectangular ratio (Br/Bs) has a big disaccommodation of core loss although it has a small core loss, and it was difficult to obtain a material without these drawbacks.